In recent years, a water absorbent resin is widely used as a constituent material, i.e., a water absorbing agent of sanitary materials, such as disposable diapers, sanitary napkins, and incontinent pads, for the purpose of absorbing body fluids. Well-known examples of the water absorbent resin are as follows: crosslinked partially neutralized polyacrylic acid; a hydrolyzed starch-acrylic acid graft polymer; a saponified vinyl acetate-acrylic ester copolymer; a hydrolyzed acrylonitrile copolymer or a crosslinked acrylonitrile copolymer; a hydrolyzed acrylamide copolymer or a crosslinked acrylamide copolymer; and a crosslinked cationic monomer. Among them, the crosslinked partially neutralized polyacrylic acid is especially preferably used in view of its properties and cost.
The crosslinked partially neutralized polyacrylic acid can be prepared by polymerizing a monomer component containing acrylic acid salt (acrylate). The acrylic acid salt can be prepared by a neutralization reaction of acrylic acid and a basic material. The neutralization is performed by mixing the acrylic acid and the basic material in a neutralization tank. The neutralization may be performed by mixing the acrylic acid and the basic material by use of a line mixer. The neutralization reaction generates neutralization heat. An acrylic acid aqueous solution in the neutralization tank is circulated through a heat exchanger so that a temperature of the acrylic acid aqueous solution can be adjusted. The aqueous solution is continuously supplied to a polymerization system through a pipe. Finally, in the polymerization system, the acrylic acid (salt) causes a polymerization reaction. Such a neutralization step is disclosed in U.S. Pat. No. 5,210,298, U.S. Patent Application Publication No. 2008/242816, International Publication No. WO 2007/028747 pamphlet, and U.S. Patent Application Publication No. 2008/194863.
In some sanitary products, the particulate water absorbing agent is used in such a manner that the particulate water absorbing agent is combined with white pulp. In view of this, the use of a particulate water absorbing agent having excellent whiteness for a sanitary product gives a user less uncomfortable feeling when he/she uses it. From the viewpoint, the particulate water absorbing agent is required to be white. In other words, it is necessary to prevent the particulate water absorbing agent from being colored.
In order to achieve the improvement of whiteness and the prevention of coloring of the particulate water absorbent resin, various water absorbent resins and techniques were proposed as follows: a water absorbent resin having excellent whiteness, which is produced from a monomer component containing acrylic acid (salt) and a given amount of methoxyphenol (International Publication No. WO 2003/051940 pamphlet); a technique for preventing coloring of a water absorbent resin by controlling hydroquinone in acrylic acid to be 0.2 wt ppm or less (U.S. Pat. No. 6,444,744); and a technique for preventing coloring of a water absorbent resin by adding a reducing agent (International Publication No. WO 2000/55245 pamphlet).
However, particulate water absorbing agents prepared by continuous polymerization often have different degrees of whiteness. Since demand for the whiteness of the particulate water absorbing agent is high, there is still room for improving the whiteness of the particulate water absorbing agent. Further, there have been used techniques for preventing coloring, such as: a technique in which a degree of purity of a raw material of the particulate water absorbing agent, such as acrylic acid, is highly increased; a technique in which polymerization conditions and/or drying conditions for preparing a water absorbent resin are set moderate; and a technique in which a new color protection agent (for example, a reducing agent) is used. However, such conventional techniques may cause an increase in production cost, a decrease in productivity, a decrease in security due to the use of the color protection agent, a decrease in water-absorbing property, and the like problems.
Further, some troubles may cause the polymerization system to stop working. Moreover, the polymerization system is purposely caused to stop working in some cases. When the polymerization system stops its operation, an aqueous solution remains within a pipe connecting the neutralization system and the polymerization system. The aqueous solution thus remaining in the pipe causes a polymerization reaction of the monomer component therein. Due to the polymerization reaction, gel is generated and adhered to an internal surface of the pipe. In order to remove the gel, it is necessary to dispose of the aqueous solution remaining in the pipe and wash the pipe before the polymerization system is restarted. This washing takes a lot of times and troubles. That is, the disposal of the aqueous solution and the washing of the pipe decrease the productivity of the water absorbent resin.
Citation List
Patent Literature 1
U.S. Pat. No. 5,210,298
Patent Literature 2
U.S. Patent Application Publication No. 2008/242816
Patent Literature 3
International Publication No. WO 2007/28747 pamphlet
Patent Literature 4
U.S. Patent Application Publication No. 2008/194863
Patent Literature 5
International Publication No. WO 2003/051940 pamphlet
Patent Literature 6
U.S. Pat. No. 6,444,744
Patent Literature 7
International Publication No. WO 2000/55245 pamphlet